Polymerization of unsaturated compounds such as vinyl acetate, (meth)acrylates, (meth)acrylic acids and styrenes dispersed in an aqueous solvent is frequently performed in a manner such that the unsaturated compound is finely dispersed into an emulsion with use of a surface active agent and/or a water-soluble high molecular weight compound, followed by the reaction.
For the hydrophobic unsaturated compound to stably disperse in the aqueous medium, the surface active agent or water-soluble high molecular compound used in forming an emulsion must possess high hydrophilicity. The surface active agent or water-soluble high molecular weight compound used for dispersing the unsaturated compound adheres to the periphery of fine polymers of the unsaturated compound. The residual surface active agent or water-soluble high molecular weight compound adhered to the periphery of polymer particles causes a lowering of the water resistance of the resultant polymer.
When importance is placed on the water resistance of polymer produced, the stability in the reaction system is deteriorated in a manner such that the unsaturated compound in the aqueous medium shows lowered dispersion stability and the polymer particles precipitate during the reaction.
Reaction of the unsaturated compound in an emulsion state requires that the unsaturated compound is stably dispersed and that the surface active agent or water-soluble high molecular weight compound does not lower the water resistance of polymer particles even if it adheres to the periphery of polymer particles after completion of the reaction. However, it is difficult for conventional surface active agents or water-soluble high molecular weight compounds to satisfy such conflicting requirements.
To avoid the problems encountered in polymerizing the unsaturated compounds in aqueous mediums, JP-A-H08-239408 discloses polymerization of a monomer in an aqueous emulsion in the presence of a high-molecular weight protective colloid having a weight-average molecular weight of not less than 20000, wherein the high-molecular weight protective colloid comprises (I) an ethylenically unsaturated monomer having an acid group or an acid anhydride group, (II) a (meth)acrylic ester of an aliphatic alcohol having 10 or more carbon atoms, (III) a main monomer selected from C1-9 alkyl (meth)acrylates, vinyl esters of carboxylic acids having 20 or less carbon atoms, ethylenically unsaturated nitriles, vinyl halides and aliphatic hydrocarbons having 2 to 8 carbon atoms and 1 or 2 double bonds and (IV) another monomer.
JP-A-2001-122911 discloses a method of emulsion polymerization of unsaturated compounds having an ethylenically unsaturated group, wherein the method employs a protective colloid of a homopolymer of N-vinylcarboxylic amide such as N-vinylacetamide or a water-soluble copolymer of N-vinylcarboxylic amide and a compound copolymerizable with the N-vinylcarboxylic amide.
The use of the protective colloid can improve the dispersion stability of the polymer, but a problem occurs in that the resulting polymer obtained using the protective colloid has an insufficient adhesive property relative to wet surfaces.